a) Field of the Invention
The present invention relates to a keyboard system for electronic musical instruments, and more particularly to a keyboard system for electronic musical instruments each key of which has a contact switch.
b) Description of the Related Art
A keyboard type electronic musical instrument uses a keyboard for designating a pitch and the like. It is generally necessary to electrically detect a key depression in order to generate a tone signal corresponding to the depressed key. To this end, each key of the keyboard of an electronic musical instrument has a contact switch. In detecting a depressed key as well as a key speed, a two-make switch is generally used as a contact switch.
A matrix circuit of contact switches are generally used to simplify the wirings of contact switches provided for keys of a keyboard. Namely, a plurality of row lines and column lines are used to form a matrix of intersections, and a switch is connected to each intersection. One two-make switch is connected at two intersections.
FIG. 2 shows a conventional contact switch circuit for the keyboard of an electronic musical instrument. A plurality of row lines X.sub.0, . . . , X.sub.15 form power supply lines, and a plurality of signal lines (column lines) Y.sub.0, . . . , Y.sub.15 are intersecting with the row lines X.sub.0, . . . , X.sub.5. Serially connected to the column lines Y.sub.0, . . . , Y.sub.15 are pull-up resistors R.sub.0, . . . , R.sub.15 whose other ends are grounded.
At each intersection between the row lines X.sub.0, . . . , X.sub.15 and column lines Y.sub.0, . . . , Y.sub.15, a serial circuit of a diode D and a switch SW is connected. When a switch SW is closed, a voltage applied to the row line X causes current to flow from the row line X to the diode D, switch SW, pull-up resistor R, and to ground. Therefore, a voltage appears on the corresponding column line Y to generate an output signal.
If the diode D is not connected at each intersection and a plurality of switches SW are closed at the same time, current may flow reversely resulting in a possibility of erroneous recognition that a switch which is not closed actually is recognized as being closed.
For example, as shown in FIG. 2, assuming that the switches at the intersections between the row line X.sub.3 and column line Y.sub.10, row line X.sub.7 and column line Y.sub.10, and row line X.sub.7 and column line Y.sub.5 are closed at the same time when a voltage is applied to the row line X.sub.3 and diodes are not provided, then the voltage is also applied to the column line Y.sub.5 via the closed switch at the intersection between the row line X.sub.7 and column line Y.sub.5. Namely, the switch at the intersection between the row line X.sub.3 and column line Y.sub.5 is erroneously recognized as being closed. For this reason, a diode for preventing reverse current is required at each intersection.
In the contact switch circuit shown in FIG. 2, it is possible to detect the close state of each of 256 switches provided for 16.times.16 row and column lines. If a two-make switch is used for each key of a keyboard, one key is assigned to two intersections of the matrix shown in FIG. 2.
Recently, an electronic musical instrument uses a central processing unit (CPU) for its main body and another CPU for its keyboard. The keyboard CPU detects each event at the keyboard, processes the detected event in the form of proper signals, and sends them to the main CPU.